As a technique for injection molding a hollow shaped article, Japanese Patent Publication No. 14968/1982 discloses a method which comprises injecting into the cavity of a die molten synthetic resin of an amount insufficient for filling the cavity, subsequently injecting compressed gas through the same inlet into the die either independently of or in combination with molten synthetic resin until the cavity is filled to capacity. This Japanese patent publication further discloses, as typical contrivance for executing this method, an apparatus which comprises a cylinder and a piston connected integrally via a switching valve to a gas source, a pressure increasing device for aspiring the gas from the gas source while the piston is in the process of retracting and increasing the pressure of this aspirated gas and feeding out the compressed gas while the piston is in the process of advancing, an injection nozzle enclosing therein a gas nozzle connected through a check valve to the pressure increasing device, and a die adapted to permit forced contact therewith of the injection nozzle and admit the molten synthetic resin and the compressed gas both emanating from the injection nozzle.
In accordance with the method and apparatus described above, the hollow shaped article is made to take shape with the compressed gas filling up the cavity of the shaped article. In Japanese Patent Publciation No. 14968/1982, however, no particular mentioned is made as to how the compressed gas filling up the cavity of the hollow shaped article is released from the interior of the die (namely from the cavity of the hollow shaped article) prior to the removal of the hollow shaped article from the die. If the discharge of the compressed gas from within the hollow shaped article prior to the removal of the hollow shaped article should be effected by separating the injection nozzle from the die, the compressed gas entrapped in the hollow shaped article would burst out instantaneously at the moment the injection nozzle breaks away from the die. This abrupt burst of the compressed gas leads to objectionable noise and, at the same time, impairs the repeatability of shape of hollow shaped articles to be successively molded. Further since the compressed gas thus liberated necessarily finds its way into the ambient air, it is not reclaimed but wasted. The gas, depending on its particular kind, could jeopardize the workshop atmosphere.
For the purpose of alleviating the drawback mentioned above, Japanese Patent Publication No. 59,899/1986 proposes a method which resides in controlling the discharge speed of the compressed gas emanating from within the cavity of the hollow shaped article and allowing the compressed gas to be recovered in a pressure container. Specifically, this invention, by the use of an apparatus similar to the apparatus disclosed in Japanese Patent Publication No. 14,968/1982, excepting the conduit interconnecting the injection nozzle and the pressure increasing device is branched and a pressure container is connected to the branch, contemplates first recovering the compressed gas from the cavity of the hollow shaped article into the pressure container and then aspirating for re-use of the recovered compressed gas from within the pressure container while the piston of the pressure increasing device is in the process of retracting.
In case of the method and apparatus mentioned above, however, when the pressure container used in the recovery of the compressed gas from within the hollow shaped article into the pressure container is small, the compressed gas cannot be recovered in an amply large amount because the pressure inside the hollow shaped article and inside the pressure container reaches the state of equilibrium after the compressed gas has been recovered only in a very small amount. The method, therefore, requires use of the pressure container of a large capacity and entails the problem of coping with the cost of equipment and the problem of securing ample floor space for the installation of the equipment. Even when the pressure container to be used has a large capacity, the compressed gas cannot be completely prevented from remaining in the cavity of the hollow shaped article under the same pressure as in the reaction container and from being released into the ambient air when the die is opened.
Particularly since the conventional method and apparatus effects the supply of the compressed gas with a pressure increasing device formed integrally of a cylinder and a piston, the following problems ensue.
Firstly, the pressure increasing device which is integrally composed of a cylinder and a piston suffers from heavy loss of energy and entails large power consumption for driving. Particularly where the pressure of the gas is increased hydraulically, the loss due to energy conversion is large. Where the compressed gas is used in a large amount, the pressure increasing device assumes a large volume and occupies a wide floor area for its installation with a sacrifice of the function as an accessorial contrivance for the injection molding machine and with an increase in the cost of equipment.
Secondly, with the pressure increasing device which is integrally composed of a cylinder and a piston, the magnitude of pressure of the compressed gas supplied into the die and the speed of this supply of the compressed gas are not easily controlled.
Thirdly, the pressure increasing device which is integrally composed of a cylinder and a piston inevitably requires a large capacity in order to ensure rapid increase of pressure necessary for enabling the gas pressure to be increased in a short span of time for the next molding cycle.